What You Need To Know About CAS Numbers

A CAS number, also known as a CAS Registry Number, is a unique numerical identifier assigned to every chemical substance described in the open scientific literature. It is issued by the Chemical Abstracts Service (CAS), a division of the American Chemical Society. Each CAS number is designed to provide a unique, unmistakable identifier for chemical substances and can include up to 10 digits, divided into three parts by hyphens. The number is used globally to provide a consistent way to identify chemicals and is often used in regulatory documents, safety data sheets (SDS), and chemical catalogs. This system helps in reducing confusion caused by the use of different names for the same chemical substance across various industries and disciplines.

In the cosmetics industry, CAS numbers are handy for ensuring regulatory compliance, and they are useful in safety data sheets, ingredient disclosures, and regulatory filings across various global agencies. They aid cosmetic chemists in accurately identifying substances.  But as is often the way, they aren’t a perfect solution.

CAS numbers, while valuable, have several drawbacks.  CAS numbers are only assigned to substances that appear in the scientific literature or are recognized by regulatory agencies, meaning there are lots of things used in cosmetics that have yet receive a CAS number, and will not until they are well-documented.  There are some that will never do so.  For instance, plant extracts and ferments like Blueberry Fruit Extract or Lactobacillus/Aloe Barbadensis Ferment Filtrate either because they vary widely in composition and thus lack a specific CAS number – or just nobody has ever got around to registering them. Custom fragrance blends used in perfumes or skincare products are proprietary mixtures so don’t have CAS registrations. Additionally, new synthetic molecules, like certain novel peptides used in anti-aging products, might not yet be documented or characterized enough to receive a CAS number.

Another limitation is the specificity of CAS numbers. They identify a particular chemical structure but do not account for impurities, mixtures, or different isotopic compositions. This lack of detail means that a CAS number alone does not provide comprehensive information about a substance’s composition or characteristics. Moreover, CAS numbers do not offer any insight into the hazards, safety, or usage of a substance, necessitating further research and documentation to ascertain these properties.

Terpenes present particular challenges for CAS number assignment. They often have various isomers and chiral forms. This vast array of structural possibilities complicates the assignment of a unique CAS number for each specific type.

Limonene serves as an excellent example to illustrate the challenges associated with CAS numbers due to its different forms. Limonene is a common terpene found in the essential oils of citrus fruits and is known to exist mainly in two isomeric forms: D-limonene and L-limonene.

D-limonene is the more commonly encountered form, especially in citrus peels, and is often used in cosmetics, cleaning products, and fragrances. It has a distinct sweet citrus odor and is noted for its effectiveness as a solvent. L-limonene, on the other hand, has a piney, turpentine-like aroma and is less common.

The issue arises because each isomer of limonene, despite having the same molecular formula (C10H16), has a different spatial arrangement of atoms, leading to different properties and uses. This distinction necessitates separate CAS numbers: D-limonene has the CAS number 5989-27-5, while 5989-54-8 can refer to the D, L and DL forms.  Almost all the limonene used in cosmetics is D-Limonene, so 5989-27-5 would be the logical number to use.  And that indeed is what is used most of the time.  But the CosIng dictionary uses another CAS number altogether – 138-86-3 – which is strictly speaking a mixture of mainly D-Limonene with some impurities.  In other contexts this mixture is known as dipentene and it is used for all sorts of purposes, but not for cosmetics.

Finally, there are errors in the CAS database, due to its vast size and scope, can lead to data entry or classification mistakes.

So while CAS numbers get used a lot and have the comforting feel of official recognition and are often very useful indeed, they aren’t always the definitive identification most of us would like them to be.  They can sometimes lead you down the wrong path.